A common type of electric motor comprises a wire wound armature assembly rotatably disposed between a pair of diametrically opposed field magnets which generate a magnetic field through which the armature is caused to rotate when electric current is supplied to the wire wound about the armature through a commutator mounted to the armature shaft. The armature of such a motor about which the wire is wound is conventionally formed of a plurality of steel laminations stacked face to face along a portion of the armature shaft.
Typically, the laminations comprise identical flat spider-like plates, each lamination having an annular hub and a plurality of spokes, commonly T-shaped, extending radially outwardly from the hub at equally spaced intervals about the circumference of the hub. The hub of each lamination has a central hole sized to permit the lamination to be slipped onto the armature shaft. The laminations are typically stamped from a thin plate of sheet steel so as to be of uniform thickness and symmetric about a central axis extending longitudinally through the hub of the lamination and coincident with the longitudinal axis of the armature shaft when the laminations are stacked onto the armature shaft. Each T-shaped spoke has a radially extending stem and a cross arm extending circumferentially from the outboard end of the stem. Wire is wrapped tightly about the stems of the stacked laminations to form the armature winding, the wire being coated with an insulating layer, typically a polymer coating, to provide insulation between each of the wire windings and between the wire windings and the laminations.
In certain applications, such as for example on electric motors used to drive a brake fluid pump in a typical automotive antilock brake system, an eccentric secondary shaft, also referred to as an eccentric cam, is formed on the outboard end of the primary armature shaft for interfacing with the brake fluid pump. The armature shaft is integrally formed of the primary shaft portion on which the lamination stack is mounted and the secondary shaft portion extending eccentrically from one end of the primary shaft portion. The secondary shaft portion extends along a longitudinal axis which is parallel to, but displaced radially from, the longitudinally extending central axis of the primary shaft portion of the armature shaft. The presence of the eccentric shaft causes the armature assembly to be unbalanced. If uncompensated, this unbalance results in unacceptable noise and potentially damaging vibration when the armature rotates during operation of the motor.
Accordingly, it is common practice to add significant amounts of a material, such as a putty or a lead loaded epoxy, or some other form of a counterweight, at one or more selected locations on the armature assembly in an attempt to nominally counterbalance the effects of the eccentricity of the secondary shaft before the armature assembly is wound with wire and the commutator is added. After final assembly of the motor, the dynamic balance is fine tuned, commonly by shaving small amounts of material off the outer circumferential surface of the stack of lamination plates, thus ensuring a dynamically balanced motor thereby reducing vibration and noise during operation. However, without the initial balancing of the armature assembly per se by adding material, the amount of material that would typically have to be removed from the outer surface of the lamination stack during dynamic balancing would be excessive and prohibitive. Nevertheless, although necessary, the initial balancing of the armature assembly itself to nominally balance the armature assembly is time consuming and labor intensive, adds undesirable weight to the motor, and is often less than completely effective. Further, there is always the possibility that the added material or counterweight may become dislodged from the armature during the operational life of the motor and subsequently damage the motor.